If House wanted to successfully shoot down any incoming missiles, he would need to position the laser system a ways away from Vegas (airbursting a hundred nukes over Vegas isn't a very strategic move), and directly under the flightpath. In the Big Empty, maybe?
I don't think they are on the Lucky 38, because by the time they are in line of sight then its too late, frankly.
If memory serves, the lasers were a last resort for the few that his networked mainframes couldnt disarm, the majority he states, his networked mainframe predicted and force transmitted subset disarm codes, likely making them little more than useless hunks of metal when they hit the ground. (I don't know much about nuclear warheads so they might not just fall harmlessly to space.
Also, if his lasers were seen in Mexico City, his weapons are firing into space, since the earth is round, you likely wouldnt see the lasers at lasers a few hundred feet in sky, they were likely akin to Hercules Bradley, the weapon the Enclave used in Broken Steel. Truth is, apart from a little dialogue from two Pre-War figures, we don't know much about Vegas's sparing of the bombs, likely done to keep an air of mystery.
Edit: Also, who says the lasers arent on a rotation platform?
@Jolipinator- We arent taking 'a few peoples words' on this, we're going by the fact that the Vegas region of the Mojave is largely intact, with 4-5 spots where nuclear warheads in outlying areas hit.